Breast cancer is associated with the loss of function of tumor suppressor proteins as well as the activation of oncogenes. About 4 percent of all breast cancer cases are associated with the inheritance of mutations at the BRCA1 gene. The BRCA1 gene carries mutations in over half of the cases among families with increased occurrence of breast and/or ovarian cancer. Among sporadic cases of breast cancer, expression of BRCA1 is reduced or undetectable in high grade ductal carcinomas, suggesting that this is a critical tumor suppressor in the etiology of breast cancer. Research in a number of laboratories has indicated that BRCA1 functions in a number of pathways, and our experiments have identified at least three separate protein complexes with which BRCA1 associates. This project will identify components of the three, or more, BRCA1-containing protein complexes, identify domains in BRCA1 which are critical for association with these complexes, and identify domains which are critical for BRCA1 function in transcription, DNA repair, and growth suppression. In this way, these experiments will correlate BRCA1 protein domains to protein complexes and to functional effects. Since BRCA1 is a multifunctional protein, it is critical to dissect the protein domains required for each function. Such structure-function evaluation can then be applied into identifying the key tumor suppressor activity (activities) of BRCA1. This project will clarify key factors in the function of BRCA1 as a tumor suppressor, and it will shed light on the breast and ovarian specificity of BRCA1 mutation.